Theoretical view on the origin and implications of structural distortions in polyoxometalates

Structural features of polyoxometalates (POMs) —versatile inorganic clusters of academic and technological interest— are discussed in the present article. POMs are, in general, very regular structures presenting a high symmetry in most cases. Distortions are, however, important for some electronic and magnetic properties. We herein discuss some particular geometric features that are crucial for the theoretical treatment and comprehension of well-known experimental phenomena. For instance, we have been able to understand and rationalize the geometrical distortions present in molybdenum POMs. Moreover, we can affirm that these geometrical distortions are caused by a pseudo Jahn Teller effect. In what concerns NMR chemical shifts, we present a discussion on the importance of geometry for the correct description of the signals and the key role played by the interatomic distances. Finally, a study on the adsorption of Keggin clusters on silver surfaces shows how the POM structure looses its regular shape to adapt to that new situation

A DFT+U study of the structural, electronic, magnetic, and mechanical properties of cubic and orthorhombic SmCoO3

SmCoO3 is a perovskite material that has gained attention as a potential substitute for La1−xSrxMnO3−d as a solid oxide fuel cell cathode. However, a number of properties have remained unknown due to the complexity of the material. For example, we know from experimental evidence that this perovskite exists in two different crystal structures, cubic and orthorhombic, and that the cobalt ion changes its spin state at high temperatures, leading to a semiconductor-to-metal transition. However, little is known about the precise magnetic structure that causes the metallic behavior or the spin state of the Co centers at high temperature. Here, we therefore present a systematic DFT+U study of the magnetic properties of SmCoO3 in order to determine what magnetic ordering is the one exhibited by the metallic phase at different temperatures. Similarly, mechanical properties are difficult to measure experimentally, which is why there is a lack of data for the two different phases of SmCoO3. Taking advantage of our DFT calculations, we have determined the mechanical properties from our calculated elastic constants, finding that both polymorphs exhibit similar ductility and brittleness, but that the cubic structure is harder than the orthorhombic phase

CO2 and H2 Adsorption and Reaction at Nin/YSZ(111) Interfaces: A Density Functional Theory Study

To recycle CO2 into sustainable fuels and chemicals, coelectrolysis of CO2 and H2O can be achieved in solid oxide electrolysis cells, where the molecules are supplied to the Ni/YSZ electrode (YSZ = yttria-stabilized zirconia). Oxygen diffusion along the electrode has been identified as the critical step in the process, where YSZ is the common catalyst support. We have investigated the interaction of a CO2 molecule with the clean YSZ(111) surface and with Nin/YSZ(111) (n = 1, 4–7, 10, and 20) interfaces, using a spin-polarized density functional theory and a long-range dispersion correction. Here, we have considered up to six initial adsorption sites and two orientations for the CO2 molecule, which showed that the adsorption is stronger at the Nin/YSZ(111) (n = 4–7, 10, and 20) interface than on the clean YSZ(111) and Ni1/YSZ(111) systems. Additionally, we have determined that the preferential adsorption site of CO2 is at the interface between the Ni clusters and the YSZ(111) surface. We have observed a bending and stretching of the molecule, demonstrating its activation upon adsorption, because of charge transfer between the metal cluster and the molecule and a mixing between Ni orbitals and CO2 orbitals. In this work, we show that although the electronic structure of the clusters depends on the cluster size, the interaction strength of CO2 with the interface is independent of the size of the supported nickel particle. Finally, we have considered the reverse water gas shift reaction and determined the hydrocarboxylic intermediate in the reaction mechanism over Ni5/YSZ(111)

CO2 and H2 adsorption and reaction at Nin/YSZ(111) interfaces: a density functional theory study

To recycle CO2 into sustainable fuel and chemicals, co-electrolysis of CO2 and H2O can be achieved in solid oxide electrolysis cells, where the molecules are supplied to the Ni/YSZ electrode (YSZ = yttria-stabilized zirconia). Oxygen diffusion along the electrode has been identified as the critical step in the process, where YSZ is the common catalyst support. We have investigated the interaction of a CO2 molecule with the clean YSZ(111) surface and with Nin/YSZ(111) (n =1, 4-7, 10, 20) interfaces, using spin polarized density functional theory (DFT) and long-range dispersion correction. Here, we have considered up to six initial adsorption sites and two orientations for the CO2 molecule, which showed that the adsorption is stronger at the Nin/YSZ(111) (n =4-7, 10, 20) interface than on the clean YSZ(111) and Ni1/YSZ(111) systems. Additionally, we have determined that the preferential adsorption site of CO2 is at the interface between the Ni clusters and the YSZ(111) surface. We have observed a bending and stretching of the molecule, demonstrating its activation upon adsorption, due to charge transfer between the metal cluster and the molecule and a mixing between Ni orbitals and CO2 orbitals. In this work, we show that, although the electronic structure of the clusters depends on the cluster size, the interaction strength of CO2 with the interface is independent of the size of the supported nickel particle. Finally, we have considered the reverse water gas shift reaction and determined the hydrocarboxylic intermediate in the reaction mechanism over Ni5/YSZ(111)

Ab initio study of vacancy formation in cubic LaMnO3 and SmCoO3 as cathode materials in solid oxide fuel cells

© 2016 Author(s). Doped LaMnO3 and SmCoO3 are important solid oxide fuel cell cathode materials. The main difference between these two perovskites is that SmCoO3 has proven to be a more efficient cathode material than LaMnO3 at lower temperatures. In order to explain the difference in efficiency, we need to gain insight into the materials' properties at the atomic level. However, while LaMnO3 has been widely studied, ab initio studies on SmCoO3 are rare. Hence, in this paper, we perform a comparative DFT + U study of the structural, electronic, and magnetic properties of these two perovskites. To that end, we first determined a suitable Hubbard parameter for the Co d-electrons to obtain a proper description of SmCoO3 that fully agrees with the available experimental data. We next evaluated the impact of oxygen and cation vacancies on the geometry, electronic, and magnetic properties. Oxygen vacancies strongly alter the electronic and magnetic structures of SmCoO3, but barely affect LaMnO3. However, due to their high formation energy, their concentrations in the material are very low and need to be induced by doping. Studying the cation vacancy concentration showed that the formation of cation vacancies is less energetically favorable than oxygen vacancies and would thus not markedly influence the performance of the cathode

A computational study of the electronic properties, ionic conduction, and thermal expansion of Sm1−xAxCoO3 and Sm1−xAxCoO3−x/2 (A = Ba2+, Ca2+, Sr2+, and x = 0.25, 0.5) as intermediate temperature SOFC cathodes

The substitutional doping of Ca2+, Sr2+, and Ba2+ on the Sm-site in the cubic perovskite SmCoO3 is reported to improve both electronic and ionic conductivities for applications as solid oxide fuel cell (SOFC) cathodes. Hence, in this study we have used density functional theory (DFT) calculations to investigate dopant configurations at two different dopant concentrations: 25 and 50%. To preserve the electroneutrality of the system, we have studied two different charge compensation mechanisms: the creation of oxygen vacancies, and electronic holes. After examining the electronic structure, charge density difference, and oxygen vacancy formation energies, we concluded that oxygen vacancy charge compensation is the preferred mechanism to maintain the electroneutrality of the system. Furthermore, we found that the improvement of the electronic conduction is not a direct consequence of the appearance of electron holes, but a result of the distortion of the material, more specifically, the distortion of the Co–O bonds. Finally, molecular dynamics were employed to model ionic conduction and thermal expansion coefficients. It was found that all dopants at both concentrations showed high ionic conduction comparable to experimental results

Computational modelling of polyoxometalates adsorbed on metallic surfaces

Aquesta tesi està dedicada a la modelització de l’adsorció de polioxometal•lats (POMs) en superfícies metàl•liques. A partir de dades estructures publicades, es plantegen diversos models que, mitjançant metodologies tant quàntiques com clàssiques, pretenen simular l’adsorció de [α-SiW12O40]4- sobre superfícies de plata. Primer s’aconsegueix reproduir el mode d’adsorció i paràmetres estructurals, així com informació espectroscòpica del sistema. Per tal de poder descriure correctament l’estructura electrònica cal introduirmolècules de solvent explícites (aigua) i els contraions (K+ i Cs+) al model. Un cop inclosos, s’aconsegueix reproduir la reduccióespontàniament del POM. En una tercera etapa es realitza un estudi comparatiu dePOMsisoestructurals de diferent càrrega i metall, que posen de manifest la necessitat d’optimitzar els sistemes incloent solvent i contraions per tal de descriure correctament les tendències esperades. Finalment, es planteja una modificació de l’estratègia dissenyada per estudiar sistemes altament carregats sobre nanopartícules d’or.This Thesis is devoted to model the adsorption of Polyoxometalates (POMs) on metallic surfaces. From the structural determination of these composite materials, we propose different models in which the combination of both quantum and classical methodologies, we intent to describe the adsorption of [α-SiW12O40]4- on silver surfaces. Adsorption mode and structural parameters are successfully reproduced, and the spectroscopic data as well. In order to correctly describe the electronic structure, it is compulsory to include explicit water molecules (water) and counterions (K+ and Cs+) in the model. After including the environment, it is possible to reproduce the spontaneous reduction of the POM. Using this strategy a comparative study of metal and charge effects in the adsorption is done. For high charged systems becomes necessary to finally optimise the structure considering the environment for a better description of the adsorption. The latest part of the Thesis is dedicated to the adsorption on gold nanoparticles

CCDC 1404438: Experimental Crystal Structure Determination

Related Article: Manuel N. Chaur, Xavier Aparicio-Anglès, Brandon Q. Mercado, Bevan Elliott, Antonio Rodríguez-Fortea, Anna Clotet, Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyen|2010|J.Phys.Chem.C|114|13003|doi:10.1021/jp104352d,An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures